An array radiation detector comprises an array of pixels and an electronic circuit forming a read means. Each pixel comprises a photosensitive element that generates electrical charges in proportion to the received quantity of photons. These electrical charges, which are also called photocharges, are processed by the read means in order to provide a piece of information that is representative of the quantity of photons that is received by each photosensitive element. The use of CMOS technology has allowed the read means to be integrated at each pixel. Thus, the electrical charges can be converted into digital signals actually inside the pixels in order to facilitate transfer of the result of the detection to the outside of the array. A common solution for producing the read means is to use a circuit that operates by integrating the electrical charges. This integration circuit comprises an integration capacitance receiving the charges from the photosensitive element, a threshold comparator, a counter and a counter-charge injection circuit. During an exposure phase, the arrival of electrical charges on the integration capacitance brings about a decrease in the voltage at the terminals thereof. During a phase of reading the electrical charges, the threshold comparator toggles a certain number of times, as long as the voltage at the terminals at the integration capacitance is below a threshold voltage. Each toggling of the comparator increments the counter by one unit and prompts the injection circuit to inject a packet of counter-charges, the quantity—Q0 of which is calibrated. The minus sign is used arbitrarily in order to indicate that the injected counter-charges have a polarity that is opposite that of the charges received from the photosensitive element. The counter is thus incremented by the number of charge packets that is necessary in order to bring back a voltage that is higher than the threshold voltage at the terminals of the integration capacitance. In practice, counter-charges are generally injected as photocharges are collected, with a counter determining the number of times the comparator toggles, in order to estimate the total quantity of charges that is injected. The reading then corresponds to the reading of the content of the counters. The number of incrementations of the counter provides a numerical value that is representative of the quantity of photons that is received by the photosensitive element.
The counter-charge injection circuit is a critical element in the integration circuit. This is because the precision of the measurement rests on the calibration of the quantity—Q0 of counter-charges. Firstly, the quantity—Q0 of counter-charges must be relatively small since it corresponds to the pace of the quantification of the charges; secondly, this quantity must be identical for each packet of counter-charges since it quantifies the charges received by the integration capacitance.
In counter-charge injection circuits today, however, the quantity of charges that is injected whenever the comparator toggles can fluctuate. The reason is that these circuits have field-effect transistors, the channels of which are affected by random noise that is referred to as its “Random Telegraph Signal”. The random nature of this noise has an influence on the quantity of counter-charges that is injected: some injections are affected by this noise, but others are not. Therefore, when wishing to estimate the total charge injected by a certain number of injections, the injections that have or have not been affected are not known.
By way of example, a counter-charge injection circuit frequently comprises two field-effect transistors (FET) connected in series and a capacitor connected between the connection point for the transistors and a fixed voltage, for example ground. A first transistor allows the capacitor to be charged to a first voltage value, referred to as charging voltage, which is controlled by the gate voltage for this transistor. The second transistor allows the capacitor to be discharged to a second voltage value, referred to as discharge voltage, which is controlled by the gate voltage of this transistor. The quantity—Q0 of counter-charges that are injected from the capacitor to the integration capacitance of the integration circuit is based on the value of the capacitance of the capacitor and on the difference between the charging and discharge voltages. However, the charging and discharge voltages cannot be deduced directly from the gate voltages of the transistors. The charging and discharge voltages correspond to the internal potentials of the transistors, which are not known precisely on account of the rts noise caused by the entrapment of charges in the channel of each transistor. This rts noise is all the more significant because the components have small dimensions in order to generate relatively small quantities—Q0 of counter-charges. In practice, this rts noise modifies the value of the quantity—Q0 by a few percent. This modification has direct repercussions on the evaluation of the quantity of photons that is received, and therefore the quality of the image obtained. Such an error is generally unacceptable in detectors, particularly in the field of medical imaging.